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Abstract:

A headlamp 1 includes: laser diodes 2 that emit excitation light; and a
light emitting part 5 that emits light upon receiving the excitation
light emitted from the laser diodes 2, the light emitting part 5
containing a first fluorescent material and a second fluorescent
material, the first fluorescent material having its emission spectrum
peak in a range of not less than 500 nm but not more than 520 nm, the
second fluorescent material having an emission spectrum peak which is
different from the emission spectrum peak of the first fluorescent
material. In a spectrum of the light emitted from the light emitting part
5, a luminous intensity at the emission spectrum peak of the first
fluorescent material is higher than a luminous intensity in an emission
spectrum covering a range of not less than 540 nm but not more than 570
nm. This allows the headlamp 1 to emit illumination light which achieves
a high visibility of an irradiation target at least in a dark place.

Claims:

1. An illuminating device comprising: an excitation light source that
emits excitation light; and a light emitting part that emits light upon
receiving the excitation light emitted from the excitation light source,
the light emitting part containing a first fluorescent material and a
second fluorescent material, the first fluorescent material having its
emission spectrum peak in a range of not less than 500 nm but not more
than 520 nm, the second fluorescent material having an emission spectrum
peak which is different from the emission spectrum peak of the first
fluorescent material, in a spectrum of the light emitted from the light
emitting part, a luminous intensity at the emission spectrum peak of the
first fluorescent material being higher than a luminous intensity in an
emission spectrum covering a range of not less than 540 nm but not more
than 570 nm.

2. The illuminating device as set forth in claim 1, wherein the first
fluorescent material contains Ce3+ as its luminescence center.

3. The illuminating device as set forth in claim 1, wherein the second
fluorescent material has its emission spectrum peak in a range of not
less than 600 nm but not more than 680 nm.

4. The illuminating device as set forth in claim 1, wherein the
excitation light source emits excitation light having a wavelength of not
less than 400 nm but not more than 420 nm.

5. The illuminating device as set forth in claim 1, wherein the first
fluorescent material is Caα-SiAlON (silicon aluminum oxynitride):Ce
fluorescent material.

6. The illuminating device as set forth in claim 1, wherein the first
fluorescent material is a nanoparticle fluorescent material containing a
III-V group compound semiconductor.

7. The illuminating device as set forth in claim 1, wherein the second
fluorescent material is CaAlSiN3:Eu fluorescent material.

8. The illuminating device as set forth in claim 1, wherein the second
fluorescent material is Sr.sub.0.8Ca.sub.0.2AlSiN3:Eu fluorescent
material.

9. A vehicle headlamp comprising an illuminating device recited in claim
1, a color of light which is emitted from the light emitting part being a
white color which falls within a legally-stipulated range of colors of
light of vehicle headlamps.

[0002] The present invention relates to an illuminating device including:
an excitation light source; and a light emitting part that emits
fluorescence responsive to excitation light from the excitation light
source. The present invention particularly relates to a vehicle headlamp.

BACKGROUND ART

[0003] Recently, vehicle headlamps have been put to practical use each of
which utilizes a white LED (Light Emitting Diode) which is a combination
of a blue light emitting diode and a fluorescent material. The adoption
of light emitting diodes makes it possible to achieve overwhelmingly
longer life of the vehicle headlamps than halogen lamps and HID (High
Intensity Discharge) lamps, which are conventional light sources.
Furthermore, it is considered that power consumption of the vehicle
headlamps can be reduced further lower than the HID lamps in the future.

[0004] Patent Literature 1 discloses one example of such vehicle
headlamps. The vehicle headlamp disclosed in Patent Literature 1 has a
plurality of LED chips which emit rays of light having respective
different colors. More specifically, Patent Literature 1 discloses that a
blue green LED or a green LED is added to the arrangement in which white
light is obtained by combining a blue LED with a fluorescent material.
Patent Literature 1 discloses only 530 nm (green) as a specific
wavelength of such additional LEDs.

[0005] A human senses light at photoreceptor cells in his retinas. The
photoreceptor cells encompass cone cells and rod cells, which are
different in light sensitivity. A sense of vision in a circumstance under
a sufficient amount of light (i.e., in a bright place) is referred to as
photopic vision. In the case of the photopic vision, the cone cells
function to recognize mainly colors and shapes. On the other hand, a
sense of vision in a dark place is referred to as scotopic vision. In the
case of the scotopic vision, the rod cells function to recognize mainly
the variations of brightness.

[0006] The photopic vision has the highest sensitivity to yellow-green
light having a wavelength of 555 nm. On the other hand, the scotopic
vision has the highest sensitivity to light having a wavelength of 507 nm
which is slightly bluish. That is, the photopic vision and the scotopic
vision have respective different peak wavelengths of luminosity factors,
and the peak wavelength of the luminosity factors of the scotopic vision
is shifted toward shorter wavelengths, with respect to that of the
photopic vision. This phenomenon is referred to as the Purkinje
phenomenon.

[0007] Patent Literature 2 discloses a retroreflector which is made in
view of the Purkinje phenomenon. The base material of the retroreflector
is blue, and the colored transparent layer thereof is yellow green.
Accordingly, in bright hours such as daytime and early dusk, the
retroreflector appears yellow green corresponding to a high photopic
relative luminosity factor. On the other hand, in the darkness of night,
the retroreflector appears blue (wavelength of close to 507 nm)
corresponding to a high scotopic relative luminosity factor, due to light
of a headlamp. Thus, the retroreflector allows proper visual guidance any
time day or night.

[0010] In general, conventional illumination light sources such as white
LEDs are made on the premise of the photopic vision. In the case of the
photopic vision, it is possible to properly distinguish colors. In other
words, the photopic vision is a sensory state where colors can be
properly distinguished. It is a natural demand that a general
illumination device provide brightness to the extent that colors can be
distinguished.

[0011] The following describes a problem of a conventional white LED. FIG.
9 is a graph showing an emission spectrum of a conventional white LED
which is a combination of a blue light emitting diode and a fluorescent
material.

[0012] The dashed line in the graph of FIG. 9 represents a spectrum of a
so-called pseudo white LED which is a combination of a blue LED and a
yellow fluorescent material. On the other hand, the spectrum represented
by the continuous line is a spectrum of a white LED which has a higher
color rendering characteristic than that of the pseudo white LED.

[0013] FIG. 9 shows that respective spectrum components of the white LEDs
are high in luminous intensity near a green spectrum (555 nm) where a
luminosity factor is the highest in the photopic vision. This is because
both white LEDs are made on the major premise of the photopic vision.

[0014] In the case of a vehicle having a headlamp which employs such a
white LED, light of the headlamp is not felt to be very bright at night
despite a very high specification value (luminous flux) on a catalog.
This problem does not arise in use of a conventional halogen lamp or a
conventional HID lamp. As a result of diligent study in view of this, the
inventors of the present invention found that conventional white LEDs
have such a problem due to a drop of a spectrum component near 510 nm.

[0015] In other words, the inventors found that since white LEDs which are
made on the premise of use in a bright place such as in a room put a
higher priority on brightness and efficiency in the photopic vision,
light of such white LEDs cannot be felt to be bright in a dark place such
as outdoors at night.

[0016] Further, none of the Patent Literatures discloses improving
visibility of an object in a bright place.

[0017] The vehicle headlamp of Patent Literature 1 emits green or blue
green light in the front direction of the vehicle, in addition to white
light. It follows that light of the vehicle headlamp differs in color in
part. Such an arrangement is not legally allowed in Japan. Therefore, the
vehicle headlamp of Patent Literature 1 cannot be realized at least in
Japan. Furthermore, Patent Literature 1 does not disclose a wavelength of
the green or blue green light. Accordingly, it is unclear whether or not
the headlamp of Patent Literature 1 makes it possible to eliminate the
drop of the spectrum component near 510 nm.

[0018] The present invention was made to solve the problem. An object of
the present invention is to provide an illuminating device, and
particularly, a vehicle headlamp, which emit illumination light which
achieves a high visibility of an irradiation target at least in a dark
place.

Solution to Problem

[0019] In order to attain the object, an illuminating device of the
present invention includes: an excitation light source that emits
excitation light; and a light emitting part that emits light upon
receiving the excitation light emitted from the excitation light source,
the light emitting part containing a first fluorescent material and a
second fluorescent material, the first fluorescent material having its
emission spectrum peak in a range from 500 nm to 520 nm, the second
fluorescent material having an emission spectrum peak which is different
from the emission spectrum peak of the first fluorescent material, in a
spectrum of the light emitted from the light emitting part, a luminous
intensity at the emission spectrum peak of the first fluorescent material
being higher than a luminous intensity in an emission spectrum covering a
range from 540 nm to 570 nm.

[0020] A human eye senses light at photoreceptor cells in the retina. The
photoreceptor cells work differently in bright and dark places.
Specifically, in a bright place (photopic vision): yellow green light is
felt to be brightest; Red light is also felt to be vivid therein; and on
the other hand, blue light is not felt to be very bright. In a dark place
(scotopic vision): blue green light, which has a shorter wavelength than
the yellow green light, is felt to be brighter than the yellow green
light; and red light, which has a long wavelength, is felt to be darkly.
This is a phenomenon, referred to as the Purkinje phenomenon, in which a
luminosity factor is shifted. In the scotopic vision, a human eye is most
sensitive to light having a wavelength of 507 nm.

[0021] In view of the Purkinje phenomenon, the inventors of the present
invention considered that: in nighttime, the vision of a human eye is the
scotopic vision, and therefore, by illuminating a road ahead with light
containing a broad blue-green spectrum, a person in a vehicle can see an
object (obstruction) on the road more clearly. In other words, in
nighttime in which the vision of a viewer is the scotopic vision, a
luminance of a light source, which is typified by a light flux (lumen)
which is usually evaluated for the photopic vision, does not always match
a sensory luminance that the viewer senses (i.e., the viewer does not
feel that the light is bright), even if the luminance of the light source
is high. Note that "can see an object more clearly" means that
distinguishability of the object or of the shape (silhouette) of the
object is improved. Therefore, it is not essential that the color of the
object can be vividly recognized.

[0022] Furthermore, the inventors of the present invention considered that
not only in a dark place but also in a bright place, irradiation of light
containing a broad blue-green spectrum stimulates rod cells so that
distinguishability of the shape of an object is improved.

[0023] According to the arrangement, the light emitting part emits light
upon receiving the excitation light emitted from the excitation light
source. Thus, the illumination light is obtained. The light emitting part
contains the first and second fluorescent materials. Since the emission
spectrum peak of the first fluorescent material is not less than 500 nm
but not more than 520 nm, the light emitted from the light emitting part
has at least one peak in the range.

[0024] Further, in the spectrum of the light emitted from the light
emitting part, a luminous intensity at the emission spectrum peak of the
first fluorescent material is higher than a luminous intensity in an
emission spectrum covering a range of not less than 540 nm but not more
than 570 nm.

[0025] In other words, the luminous intensity at that emission spectrum
peak of the first fluorescent material which is located near the peak of
the luminosity factor in the scotopic vision is higher than the luminous
intensities in the emission spectrum in the range of not less than 540 nm
but not more than 570 nm within which range the luminosity factor in the
photopic vision is peaked.

[0026] This allows the light emitting part to emit light which achieves a
high luminosity factor in the scotopic vision. As a result, it is
possible to improve visibility of an object irradiated by the
illuminating device in a dark place.

[0027] It is considered that irradiation of light having a wavelength in
the range of not less than 500 nm but not more than 520 nm stimulates rod
cells which are involved in recognition of the shape of an object so that
visibility of an object is improved in a bright place. Therefore, the
technical scope of the present invention encompasses not only
illuminating devices which are used in a dark place, but also the
aforementioned illuminating device which is used in a bright place.
However, the present invention is not limited to illuminating devices
which make it possible to improve visibility of an object both in a dark
place and a bright place. That is, the illuminating device of the present
invention makes it possible to improve at least visibility of an object
in a dark place.

Advantageous Effects of Invention

[0028] As described above, the illuminating device of the present
invention includes an excitation light source that emits excitation
light; and a light emitting part that emits light upon receiving the
excitation light emitted from the excitation light source, the light
emitting part containing a first fluorescent material and a second
fluorescent material, the first fluorescent material having its emission
spectrum peak in a range of not less than 500 nm but not more than 520
nm, the second fluorescent material having an emission spectrum peak
which is different from the emission spectrum peak of the first
fluorescent material, in a spectrum of the light emitted from the light
emitting part, a luminous intensity at the emission spectrum peak of the
first fluorescent material being higher than a luminous intensity in an
emission spectrum covering a range of not less than 540 nm but not more
than 570 nm.

[0029] This makes it possible to emit light which achieves a high
luminosity factor in the scotopic vision, and to improve visibility of an
object irradiated by the illuminating device at least in a dark place.

[0043]FIG. 7 is a cross-sectional view schematically illustrating an
arrangement of a headlamp of another embodiment of the present invention.

[0044] FIG. 8

[0045] FIG. 8 is a view illustrating positional relation between exit end
parts of optical fiber and the light emitting part.

[0046] FIG. 9

[0047] FIG. 9 is a graph showing an emission spectrum of a conventional
white LED which is a combination of a blue light emitting diode and a
fluorescent material.

DESCRIPTION OF EMBODIMENTS

Embodiment 1

[0048] The following describes one embodiment of the present invention,
with reference to FIGS. 1 to 3.

[0049] (Technical Idea of Present Invention)

[0050] In view of the Purkinje phenomenon, the inventors of the present
invention considered that: in nighttime, the vision of a human eye is the
scotopic vision, and therefore, by illuminating a road ahead with light
containing a broad blue-green spectrum, a person in a vehicle can see an
object (obstruction) on the road more clearly. In other words, in
nighttime in which the vision of a viewer is the scotopic vision, a
luminance of a light source, which is typified by a light flux (lumen)
which is usually evaluated for the photopic vision, does not always match
a sensory luminance that the viewer senses (i.e., the viewer does not
feel that the light is bright), even if the luminance of the light source
is high. Note that "can see an object more clearly" means that
distinguishability of the object or of the shape (silhouette) of the
object is improved. Therefore, it is not essential that the color of the
object can be vividly recognized.

[0051] Furthermore, the inventors of the present invention considered that
not only in a dark place but also in a bright place, irradiation of light
containing a broad blue-green spectrum stimulates rod cells so that
distinguishability of the shape of an object is improved.

[0052] The illuminating device of the present invention was made based on
the technical idea. By emitting light whose luminosity factor is high
under circumstances where human vision is the scotopic vision, the
illuminating device makes it possible to improve visibility of an object
in a dark place (e.g., in night driving). Further, in some cases, the
illuminating device of the present invention makes it possible to improve
visibility of an object not only in a dark place but also in a bright
place. That is, the illuminating device of the present invention makes it
possible to improve at least visibility of an object in a dark place.

[0053] The present embodiment describes, as one example of the
illuminating device of the present invention, a headlamp (vehicle
headlamp) 1 which satisfies light distribution property standards for
driving headlamps (i.e., high beam) for automobiles. Note that the
illuminating device of the present invention may be realized as a
headlamp for a vehicle except automobiles or for a moving object except
automobiles (e.g., a human, a vessel, an airplane, a submersible vessel,
or a rocket), or may be realized as another illuminating device such as a
searchlight.

[0054] (Arrangement of Headlamp 1)

[0055] The following describes an arrangement of the headlamp
(illuminating device) 1 of the present embodiment, with reference to FIG.
1. FIG. 1 is a view schematically illustrating an arrangement of the
headlamp 1 of the present embodiment. As illustrated in FIG. 1, the
headlamp 1 includes laser diodes 2, aspheric lenses 3, a light guide
section 4, a light emitting part 5, a reflection mirror 6, and a
transparent plate 7.

[0056] (Laser Diode 2)

[0057] The laser diodes 2 function as an excitation light sources which
emit excitation light. The laser diodes 2 may be a single laser diode 2
or a plurality of laser diodes 2. Further, each of the laser diodes 2 may
be one such that one luminous point is provided on one chip, or may be
one such that a plurality of luminous points are provided on one chip.
The present embodiment deals with the laser diodes 2 in each of which one
luminous point is provided on one chip.

[0058] Each of the laser diodes 2 is arranged such that e.g.: one luminous
point (one stripe) is provided on one chip; each of the laser diodes 2
emits a laser beam at a wavelength of 405 nm (bluish purple); an optical
output is 1.0 W; an operating voltage is 5 V; and an operating current is
0.7 A. Each of the laser diodes 2 is sealed in a package (stem) that is
5.6 mm in diameter. Since 10 laser diodes 2 are used in the present
embodiment, a total optical output is 10 W. For convenience, FIG. 1
illustrates only one laser diode 2.

[0059] A wavelength of a laser beam which is emitted from each of the
laser diodes 2 is not limited to 405 nm. That is, a peak wavelength of
the laser beam is in a wavelength range of not less than 400 nm but not
more than 460 nm, more preferably, in a wavelength range of not less than
400 nm but not more than 420 nm.

[0060] By adopting, as a wavelength of the laser diodes 2, a wavelength
which has a peak wavelength in the wavelength range of not less than 400
nm but not more than 420 nm, it becomes possible to expand the range of
options to choose a second fluorescent material which is combined with a
first fluorescent material (its emission peak wavelength is in a range of
not less than 500 nm but not more than 520 nm) so that the light emitting
part 5 for emitting white light is made. Specifically, it becomes
possible to adopt, as the second fluorescent material, a fluorescent
material having an emission spectrum peak in a range of not less than 600
nm but not more than 680 nm.

[0061] In a case where the fluorescent materials of the light emitting
part 5 is an oxynitride fluorescent material, it is preferable that an
optical output of each of the laser diodes 2 be in a range of not less
than 1 W but not more than 20 W, and a light density of a laser beam
which is incident on the light emitting part 5 be in a range of not less
than 0.1 W/mm2 but not more than 50 W/mm2. Such an optical
output makes it possible to achieve a luminous flux and a luminance which
are required for a vehicle headlamp, and to prevent extreme deterioration
of the light emitting part 5 due to a high-power laser beam. In other
words, such an optical output makes it possible to realize a longer life
of a light source despite a high luminous flux and a high luminance.

[0062] Note that, in a case where a semiconductor nanoparticle fluorescent
material is adopted as the fluorescent materials of the light emitting
part 5, a light density of the laser beam which is incident on the light
emitting part 5 may be higher than 50 W/mm2.

[0063] (Aspheric Lenses 3)

[0064] The aspheric lenses 3 are lenses for guiding laser beams emitted
from the laser diodes 2 so that the laser beams enter the light guide
section 4 via a light receiving surface 4a which is one of two end
surfaces of the light guide section 4. Examples of the aspheric lenses 3
encompass FLKN1 405 manufactured by Alps Electric Co., Ltd. A shape and a
material of the aspheric lenses 3 are not particularly limited, provided
that the aforementioned function is achieved. A material of the aspheric
lenses 3 preferably has a high transmittance near 405 nm and a high heat
resistance.

[0065] The aspheric lenses 3 are for converging the laser beams emitted
from the laser diodes 2 so as to guide the laser beams to a relatively
small (e.g., diameter of not more than 1 mm) light receiving surface.
Therefore, in a case where the light receiving surface 4a of the light
guide section 4 is large to the extent that there is no need to converge
the laser beams, there is no need to provide the aspheric lenses 3.

[0066] (Light Guide Section 4)

[0067] The light guide section 4 is a light guide having a shape of a
truncated cone. The light guide section 4 converges the laser beams
emitted from the laser diodes 2 so as to guide the laser beams to the
light emitting part 5 (i.e., a laser beam-irradiated surface of the light
emitting part 5). The light guide section 4 is optically combined with
the laser diodes 2 via the aspheric lenses 3 (or directly). The light
guide section 4 has: the light receiving surface 4a (entrance end part)
for receiving the laser beams emitted from the laser diodes 2; and a
light emitting surface 4b (exit end part) for emitting, toward the light
emitting part 5, the laser beams received on the light receiving surface
4a.

[0068] The light emitting surface 4b has a smaller area than that of the
light receiving surface 4a. Accordingly, the laser beams which have
entered the light guide section 4 via the light receiving surface 4a are
converged by traveling to the light emitting surface 4b while being
reflected on a side surface of the light guide section 4. In this way,
the laser beams thus converged are emitted via the light emitting surface
4b.

[0069] The light guide section 4 is made from BK7, fused quartz, acrylic
resin, or another transparent material. The light receiving surface 4a
and the light emitting surface 4b may be a flat surface or a curved
surface.

[0070] The light guide section 4 may have a shape of a truncated pyramid,
and may be an optical fiber, provided that the light guide section 4
guides the laser beams from the laser diodes 2 to the light emitting part
5. Alternatively, it may be arranged such that the light guide section 4
is not provided but the light emitting part 5 is irradiated with the
laser beams from the laser diodes 2 directly or via the aspheric lenses
3. Such an arrangement is possible in a case where a distance between the
laser diodes 2 and the light emitting part 5 is small.

[0071] (Composition of Light Emitting Part 5)

[0072] The light emitting part 5 emits light in response to the laser
beams emitted via the light emitting surface 4b of the light guide
section 4. Specifically, the light emitting part 5 is such that a
plurality of fluorescent materials which emit light in response to a
laser beam are dispersed in a fluorescent material-holding substance
(sealing material). More specifically, the light emitting part 5 contains
a first fluorescent material and a second fluorescent material having an
emission spectrum peak which is different from that of the first
fluorescent material. The first fluorescent material has an emission
spectrum peak near 507 nm which is a peak wavelength of the luminosity
factor in the photopic vision. More specifically, the first fluorescent
material has an emission spectrum peak in a range of not less than 500 nm
but not more than 520 nm. On the other hand, the second fluorescent
material has an emission spectrum peak in a range of, e.g., not less than
600 nm but not more than 680 nm.

[0073] The composition of the light emitting part 5 is adjusted so that in
a spectrum of light which is emitted from the light emitting part 5, a
luminous intensity at the emission spectrum peak of the first fluorescent
material is higher than luminous intensities in an emission spectrum
covering a range of not less than 540 nm but not more than 570 nm.

[0074] Each of the first and second fluorescent materials is an oxynitride
fluorescent material, or a semiconductor nanoparticle fluorescent
material which contains nanometer-size particles of a III-V group
compound semiconductor.

[0075] A so-called sialon (SiAlON (silicon aluminum oxynitride))
fluorescent material can be adopted as the oxynitride fluorescent
material. The sialon fluorescent material is silicon nitride in which (i)
one or more of silicon atoms are substituted by an aluminum atom(s) and
(ii) one or more of nitrogen atoms are substituted by an oxygen atom(s).
The sialon fluorescent material can be produced by solidifying alumina
(Al2O3), silica (SiO2), a rare-earth element, and/or the
like with silicon nitride (Si3N4). The first fluorescent
material is, e.g., Caα-SiAlON:Ce3+ fluorescent material. On
the other hand, the second fluorescent material is, e.g.,
CaAlSiN3:Eu2+ fluorescent material.

[0076] The semiconductor nanoparticle fluorescent material is
characterized in that even if the nanoparticles are made of an identical
compound semiconductor (e.g., indium phosphorus: InP), it is possible to
cause the nanoparticles to emit light of different colors by changing
particle size thereof to a nanometer size. The change in color occurs due
to a quantum size effect. For example, in the case where the
semiconductor nanoparticle fluorescent material is made of InP, the
semiconductor nanoparticle fluorescent material emits red light when each
of the nanoparticles is approximately 3 nm to 4 nm in diameter. The
particle size is evaluated with use of a transmission electron microscope
[TEM].

[0077] Further, the semiconductor nanoparticle fluorescent material is a
semiconductor-based material, and therefore the life of the fluorescence
is short. Accordingly, the semiconductor nanoparticle fluorescent
material can quickly convert power of the excitation light into
fluorescence, and therefore is highly resistant to high-power excitation
light. This is because the emission life of the semiconductor
nanoparticle fluorescent material is approximately 10 nanoseconds, which
is some five digits less than a commonly used fluorescent material that
contains rare earth as a luminescence center.

[0078] In addition, since the emission life is short as described above,
it is possible to quickly repeat absorption of a laser beam and emission
of fluorescence. Accordingly, it is possible to maintain high conversion
efficiency with respect to intense laser beams, thereby reducing heat
emission from the fluorescent materials. This makes it possible to
further prevent a heat deterioration (discoloration and/or deformation)
in the light emitting part 5. This achieves a longer life of the headlamp
1.

[0079] The sealing material may be a resin such as silicon resin, or may
be a glass material (e.g., inorganic glass and organic hybrid glass). The
light emitting part 5 may be made by ramming the fluorescent materials
only. However, the light emitting part 5 is preferably such that the
fluorescent materials are dispersed in the sealing material. This is
because deterioration of the light emitting part 5 due to laser
irradiation is accelerated in a case where the light emitting part 5 is
made by ramming the fluorescent materials only.

[0080] (Disposition and Shape of Light Emitting Part 5)

[0081] The light emitting part 5 is fixed in a focal point of the
reflection mirror 6 or in the vicinity thereof, on an inner surface (on a
light emitting surface 4b side) of the transparent plate 7. A method of
fixing a position of the light emitting part 5 is not limited to this,
and therefore the light emitting part 5 may be fixed by using a
bar-shaped or tubular member extending from the reflection mirror 6.

[0082] A shape of the light emitting part 5 is not particularly limited,
but may be a rectangular parallelepiped or a cylinder. In the present
embodiment, the light emitting part 5 is a cylindrical column, which is 3
mm in diameter and 3 mm in thickness (height). The laser beam-irradiated
surface, which is a surface of the light emitting part 5 to be irradiated
with a laser beam, is not necessarily required to be a flat surface but
may be a curved surface. However, in order to control reflection of a
laser beam, it is preferable that the laser beam-irradiated surface be a
flat surface. In a case where the laser beam-irradiated surface is a
curved surface, at least an incident angle to the curved surface is
significantly different from that of the flat surface. This significantly
changes a traveling direction of the reflected light, depending on a
position irradiated with the laser beam. As a result, the control of the
reflection function of the laser beam can be difficult. In contrast, in a
case where the laser beam-irradiated surface is a flat surface, the
traveling direction of the reflected light is hardly changed even if a
position to be irradiated with the laser beam is somewhat shifted.
Therefore, it is easy to control the reflection direction. In some cases,
it is easy to put an absorber to absorb the laser beam in a position to
be irradiated with the reflected light.

[0083] Further, the light emitting part 5 is not necessarily required to
have a thickness of 3 mm. The light emitting part 5 has a thickness such
that the laser beams are wholly converted into white light by the light
emitting part 5 or such that the laser beams are sufficiently scattered
by the light emitting part 5. In other words, the light emitting part 5
has a thickness such that an intensity of coherent light harmful to human
health is decreased to a safe level, or such that the coherent light is
converted into harmless incoherent light.

[0084] A required thickness of the light emitting part 5 varies depending
on a ratio between the sealing material and the fluorescent materials in
the light emitting part 5. A higher content of the fluorescent materials
in the light emitting part 5 makes it possible to adopt a smaller
thickness of the light emitting part 5 because the higher content of the
fluorescent materials in the light emitting part 5, the higher efficiency
in the conversion of the laser beams into the white light.

[0085] (Reflection Mirror 6)

[0086] The reflection mirror 6 reflects incoherent light emitted from the
light emitting part 5, thereby forming a bundle of beams reflected at
predetermined solid angles. That is, the reflection mirror 6 reflects
light emitted from the light emitting part 5, thereby forming a bundle of
beams traveling in a forward direction from the headlamp 1. The
reflection mirror 6 is for example a member having a curved surface (cup
shape), whose surface is coated with a metal thin film. The reflection
mirror 6 has an opening, which opens toward a direction in which the
reflected light travels.

[0087] The reflection mirror 6 is not limited to a hemispherical mirror,
but may be an ellipsoidal mirror, a parabolic mirror, or a mirror having
a part of such a curved surface. That is, the reflection surface of the
reflection mirror 6 contains at least a part a curved surface which is
formed in such a manner that a figure (an ellipse, a circle, or a
parabola) is rotated around a rotation axis.

[0088] (Transparent Plate 7)

[0089] The transparent plate 7 is a transparent resin plate that covers
the opening of the reflection mirror 6 and holds the light emitting part
5. The transparent plate 7 is preferably made from a material that (i)
blocks laser beams emitted from the laser diodes 2 and (ii) transmits
white light (incoherent light) into which the light emitting part 5
converts the laser beams. The transparent plate 7 is not limited to the
resin plate but may be an inorganic glass plate or the like.

[0090] The light emitting part 5 converts most of a coherent laser beam
into incoherent white light. Note however that, part of the laser beam
may not be converted for some reasons. Even so, since the transparent
plate 7 blocks the laser beams, it is possible to prevent the laser beams
from leaking out. Note here that, in a case where (a) such an effect is
not necessary and (b) the light emitting part 5 is held by a member other
than the transparent plate 7, the transparent plate 7 may be omitted.

[0091] (Arrangement of Laser Diodes 2)

[0092] The following description discusses a fundamental structure of each
of the laser diodes 2. (a) of FIG. 2 is a circuit diagram schematically
illustrating a circuit of a laser diode 2. (b) of FIG. 2 is a perspective
view illustrating a fundamental structure of the laser diode 2. As
illustrated in (b) of FIG. 2, the laser diode 2 includes: a cathode
electrode 19, a substrate 18, a clad layer 113, an active layer 111, a
clad layer 112, and an anode electrode 17, which are stacked in this
order.

[0093] The substrate 18 is a semiconductor substrate. In order to obtain
excitation light such as from blue excitation light to ultraviolet
excitation light so as to excite a fluorescent material as in the present
invention, it is preferable that the substrate 18 be made of GaN,
sapphire, and/or SiC. Generally, for example, a substrate for the laser
diode is constituted by: a IV group semiconductor such as that made of
Si, Ge, or SiC; a III-V group compound semiconductor such as that made of
GaAs, GaP, InP, AlAs, GaN, InN, InSb, GaSb, or AlN; a II-VI group
compound semiconductor such as that made of ZnTe, ZeSe, ZnS, or ZnO;
oxide insulator such as ZnO, Al2O3, SiO2, TiO2,
CrO2, or CeO2; or nitride insulator such as SiN.

[0094] The anode electrode 17 injects an electric current into the active
layer 111 via the clad layer 112.

[0095] The cathode electrode 19 injects, from a bottom of the substrate 18
and via the clad layer 113, an electric current into the active layer
111. The electrical current is injected by applying forward bias to the
anode electrode 17 and the cathode electrode 19.

[0096] The active layer 111 is sandwiched between the clad layer 113 and
the clad layer 112.

[0097] Each of the active layer 111 and the clad layers 112 and 113 is
constituted by, so as to obtain excitation light such as from blue
excitation light to ultraviolet excitation light, a mixed crystal
semiconductor made of AlInGaN. Generally, each of an active layer and
clad layer of the laser diode is constituted by a mixed crystal
semiconductor, which contains as a main composition Al, Ga, In, As, P, N,
and/or Sb. The active layer and clad layers in accordance with the
present invention can also be constituted by such a mixed crystal
semiconductor. Alternatively, the active layer and clad layers can be
constituted by a II-VI group compound semiconductor such as that made of
Zn, Mg, S, Se, Te, or ZnO.

[0098] The active layer 111 emits light upon injection of the electric
current. The light emitted from the active layer 111 is kept within the
active layer 111, due to a difference in refractive indices of the clad
layer 112 and the clad layer 113.

[0099] The active layer 111 further has a front cleavage surface 114 and a
back cleavage surface 115, which face each other so as to keep, within
the active layer 111, light that is enhanced by induced emission. The
front cleavage surface 114 and the back cleavage surface 115 serve as
mirrors.

[0100] Note however that, unlike a mirror that reflects light completely,
the front cleavage surface 114 and the back cleavage surface 115 (for
convenience of description, these are collectively referred to as the
front cleavage surface 114 in the present embodiment) of the active layer
111 transmits part of the light enhanced due to induced emission. The
light emitted outward from the front cleavage surface 114 is excitation
light L0. The active layer 111 can have a multilayer quantum well
structure.

[0101] The back cleavage surface 115, which faces the front cleavage
surface 114, has a reflection film (not illustrated) for laser
oscillation. By differentiating reflectance of the front cleavage surface
114 from reflectance of the back cleavage surface 115, it is possible for
most of the excitation light L0 to be emitted from a luminous point 103
of an end surface having low reflectance (e.g., the front cleavage
surface 114).

[0102] Each of the clad layer 113 and the clad layer 112 can be
constituted by: a n-type or p-type III-V group compound semiconductor
such as that made of GaAs, GaP, InP, AlAs, GaN, InN, InSb, GaSb, or MN;
or a n-type or p-type II-VI group compound semiconductor such as that
made of ZnTe, ZeSe, ZnS, or ZnO. The electrical current can be injected
into the active layer 111 by applying forward bias to the anode electrode
17 and the cathode electrode 19.

[0103] A semiconductor layer such as the clad layer 113, the clad layer
112, and the active layer 111 can be formed by a commonly known film
formation method such as MOCVD (metalorganic chemical vapor deposition),
MBE (molecular beam epitaxy), CVD (chemical vapor deposition),
laser-ablation, or sputtering. Each metal layer can be formed by a
commonly known film formation method such as vacuum vapor deposition,
plating, laser-ablation, or sputtering.

[0104] (Principle of Light Emission of Light emitting part 5)

[0105] Next, the following description discusses a principle of a
fluorescent material emitting light upon irradiation of a laser beam
oscillated from the laser diode 2.

[0106] First, the fluorescent material contained in the light emitting
part 5 is irradiated with the laser beam oscillated from the laser diode
2. Upon irradiation of the laser beam, an energy state of electrons in
the fluorescent material is excited from a low energy state into a high
energy state (excitation state).

[0107] After that, since the excitation state is unstable, the energy
state of the electrons in the fluorescent material returns to the low
energy state (an energy state of a ground level, or an energy state of an
intermediate metastable level between ground and excited levels) after a
certain period of time.

[0108] As described above, the electrons excited to be in the high energy
state returns to the low energy state. In this way, the fluorescent
material emits light.

[0109] Note here that, white light can be made by mixing three colors
which meet the isochromatic principle, or by mixing two colors which are
complimentary colors for each other. The white light can be obtained by
combining (i) a color of the laser beam oscillated from the laser diode 2
and (ii) a color of the light emitted from the fluorescent material on
the basis of the foregoing principle and complementary relationship.

Example 1

[0110] The following describes an example of the light emitting part 5 in
more detail. In the present embodiment, employed as the first fluorescent
material having an emission spectrum peak in a range of not less than 500
nm but not more than 520 nm is Caα-SiAlON:Ce3+ fluorescent
material (hereinafter, abbreviated as Caα-SiAlON fluorescent
material), and employed as the second fluorescent material having an
emission spectrum peak in a range of not less than 620 nm but not more
than 680 nm is CASN:Eu (CaAlSiN3:Eu2+) fluorescent material
(hereinafter, referred to as CASN fluorescent material).

[0111] (Properties of Fluorescent Materials)

[0112]FIG. 3 is a table showing properties of the
Caα-SiAlON:Ce3+ fluorescent material and the
CaAlSiN3:Eu2+ fluorescent material. As shown in the table, the
Caα-SiAlON fluorescent material emits fluorescence ranging from
blue to green, and its emission peak wavelength is 510 nm. The
Caα-SiAlON fluorescent material has an emission half-value breadth
of 110 nm, which is broad. Thus, the Caα-SiAlON fluorescent
material fully covers wavelengths with high scotopic relative luminosity
factors. Further, the Caα-SiAlON fluorescent material has a high
luminous efficiency of 58%. Further, the Caα-SiAlON fluorescent
material has a high heat resistance. Therefore, the light emitting part 5
is unlikely to become deteriorated even if the light emitting part 5 is
irradiated with a high-power laser beam at a high light density. This
makes it possible to realize a headlamp with a high luminance and a high
luminous flux.

[0113] The CASN fluorescent material emits red fluorescent, and its
emission peak wavelength is 650 nm. The CASN fluorescent material has a
luminous efficiency of 71%, and an emission half-value breadth of 93 nm.
The CASN fluorescent material also has a high heat resistance. Therefore,
the light emitting part 5 is unlikely to become deteriorated even if the
light emitting part 5 is irradiated with a high-power excitation light at
a high light density. This makes it possible to realize a headlamp with a
high luminance and a high luminous flux.

[0114]FIG. 3 shows values obtained in a case where an excitation
wavelength was 405 nm. In a case where an excitation wavelength of the
Caα-SiAlON fluorescent material increases, an emission peak
wavelength thereof increases accordingly. This decreases an absorbance
and an internal quantum efficiency. As a result, a luminous efficiency
also decreases. In this case, a half-value breadth becomes somewhat
wider.

[0115] In contrast, in a case where the excitation wavelength decreases,
the absorptance, the internal quantum efficiency, and the luminous
efficiency somewhat increase up to approximately 350 nm. In this case, an
emission peak wavelength decreases somewhat, and a half-value breadth
also becomes somewhat narrower. In a case where the excitation wavelength
is shorter than 350 nm, the Caα-SiAlON fluorescent material does
not emit fluorescent.

[0116] In an excitation wavelength range of not less than 350 nm but not
more than 450 nm, the CASN fluorescent material has almost constant
properties (emission peak wavelength, absorptance, internal quantum
efficiency, luminous efficiency, and half-value breadth). The CASN
fluorescent material has somewhat undesirable properties in an excitation
wavelength range of not shorter than 450 nm. In an excitation wavelength
range of not longer than 350 nm, the CASN fluorescent material does not
emit fluorescent, as is the case with the Caα-SiAlON fluorescent
material.

[0117] (Adjustment of White Light)

[0118] The light emitting part 5 containing these fluorescent materials
was irradiated with the laser beams which were emitted from the laser
diodes 2 at an oscillation wavelength of 405 nm, so that illumination
light is generated. A ratio between the Caα-SiAlON fluorescent
material and the CASN fluorescent material in the light emitting part 5
was adjusted so that a color temperature of the illumination light was in
a range of not less than 3000 K but not more than 7000 K, and the
illumination light was white light which falls within a range of white
colors which are required for headlamps which range is stipulated under
the Road Trucking Vehicle Law. The color temperature was adjusted so as
to be preferred by many users in the market.

[0119]FIG. 4 is a graph showing a chromaticity range of white colors
which are required for vehicle headlamps. The chromaticity range is
stipulated in Japan by law as shown in FIG. 4. Specifically, the
chromaticity range corresponds to the inside of a polygon which has six
points 35 as its vertexes.

[0120] According to the graph, it is possible to realize chromaticities
indicated by points within a triangle 30 which connects a point 31 which
indicates an emission peak wavelength of the Caα-SiAlON fluorescent
material, a point 32 which indicates an emission peak wavelength of the
CASN fluorescent material, and a point 33 which indicates the oscillation
wavelength 405 nm of the laser diodes 2 which are excitation light
sources. A point which indicates a chromaticity of illumination light
which is realized moves within the triangle 30, by changing: a ratio
between the Caα-SiAlON fluorescent material and the CASN
fluorescent material in the light emitting part 5, a mixing ratio between
the sealing material and the fluorescent materials in the light emitting
part 5, and an intensity of the excitation light. For example, in a case
where a ratio of the Caα-SiAlON fluorescent material is increased,
a point indicating a chromaticity of the illumination light approaches
the point 31. As a result, the illumination light has a more bluish
color.

[0121] The triangle 30 contains the polygon. The ratio between the
Caα-SiAlON fluorescent material and the CASN fluorescent material
in the light emitting part 5, the mixing ratio between the sealing
material and the fluorescent materials in the light emitting part 5, and
the intensity of the excitation light are determined so that a
chromaticity is realized which is indicated by a point within the
polygon.

[0122] A chromaticity of the illumination light is determined so that a
point indicating the chromaticity is within the region defined by the
triangle which has points 31, 34a, and 34c as its vertexes, and within
the region defined by the polygon which has the points 35 as its
vertexes.

[0123] The point 34a is a point where a ratio between a radiant flux of
the fluorescent from CASN:Eu2+ and a radiant flux of the laser beams
which are emitted from the laser diodes 2 is 1:0.1. The point 34b is a
point where the ratio is 1:1. The point 34c is a point where the ratio is
1:2.5. The laser beams themselves have their own chromaticity. Therefore,
by employing a constant composition of the light emitting part 5 and
changing the radiant flux of the laser beams, a point indicating the
chromaticity of the illumination light moves on a line segment which
connects the points 32 and 33.

[0124] The ratio between the first and second fluorescent materials varies
according to respective luminous efficiencies as well as respective
fluorescence colors. An ultimate color of the illumination light varies
according also to a color and an intensity of the laser beams and a type
and an amount of the sealing material. Therefore, the ratio between the
first and second fluorescent materials is adjusted in consideration of
these factors.

[0125] The present example employed 1:3.6:100 as a ratio of the
Caα-SiAlON fluorescent material, the CASN fluorescent material, and
silicon resin which serves as the sealing material, so as to form the
light emitting part 5 having a diameter and a height of 3 mm. The light
emitting part 5 was irradiated with laser beams having a wavelength of
405 nm, in order to measure a spectrum and a chromaticity of obtained
illumination light.

[0126] As a result, the chromaticity of the illumination light was one
indicated by coordinates of x=0.4101 and y=0.4017 in the graph of FIG. 4.
The chromaticity satisfies a safety standard in Japan for road trucking
vehicles. In other words, the measurement demonstrated that a color of
the light emitted from the light emitting part 5 was adjusted to a white
color within the legally-stipulated range of colors of light of vehicle
headlamps. A color temperature of the illumination light was 3500 K. An
average color rendering index Ra was 86.6. A special color rendering
index R9 was 57.6.

[0127] FIG. 5 is a graph showing an emission spectrum of the light
emitting part 5 of the present example. An emission spectrum peak of the
Caα-SiAlON fluorescent material falls within a wavelength range of
not less than 500 nm but not more than 520 nm. The emission spectrum peak
locates near a peak of the luminosity factor in the scotopic vision. As
shown in FIG. 5, this made it possible to obtain an emission spectrum
which has a sufficiently high intensity near 510 nm around which the
luminosity factor is peaked in the scotopic vision. In the spectrum of
the light emitted from the light emitting part 5, a luminous intensity at
the emission spectrum peak of the Caα-SiAlON fluorescent material
is higher than luminous intensities in an emission spectrum covering a
range of not less than 540 nm but not more than 570 nm. In other words,
the luminous intensity at the emission spectrum peak of the
Caα-SiAlON fluorescent material which is the first fluorescent
material is higher than the luminous intensities in the emission spectrum
covering the range of not less than 540 nm but not more than 570 nm
within which range the peak of luminosity factors in the photopic vision
falls.

[0128] As a result, employment of the white light source as a vehicle
headlamp makes it possible to realize a vehicle headlamp which excels in
obstruction visibility in night driving in which human vision is the
scotopic vision.

[0129] Further, in a bright place, irradiation of light having a
wavelength in the range of not less than 500 nm but not more than 520 nm
(particularly, light having a wavelength close to 507 nm) stimulates rod
cells which are involved in recognition of the shape of an object so that
visibility of an object is improved. Therefore, even if vision is not the
scotopic vision totally, this makes it possible to realize a headlamp
which excels in obstruction visibility in a case where vision lies
between the scotopic vision and the photopic vision.

[0130] The peak near 510 nm was very broad. This makes it possible to
realize a vehicle headlamp whose brightness cannot be felt by a user to
be discontinuous in a case where a luminosity factor varies from early
evening (photopic vision) in which dim light still remains to dark night
(scotopic vision).

[0131] Further, the white light source has an excellent average color
rendering index of 86.6. This allows a user to visually recognize various
road signs clearly in night driving.

[0132] Since the ratio between the first and second fluorescent materials
is merely an example, the present invention is not limited to the ratio.

Example 2

[0133] The following describes another example of the light emitting part
5. As is the case with the Example 1, the present example employed the
Caα-SiAlON fluorescent material and the CASN fluorescent material
as the first and second fluorescent materials, respectively. However, in
the present example, the light emitting part 5 having a diameter of 3 mm
and a height of 5 mm was formed at the ratio 1:3.6:250 of the
Caα-SiAlON fluorescent material, the CASN fluorescent material, and
the silicon resin which serves as the sealing material. The light
emitting part 5 was irradiated with laser beams having a wavelength of
405 nm, in order to measure a spectrum and a chromaticity of obtained
illumination light.

[0134] As a result, the chromaticity of the illumination light was one
indicated by coordinates of x=0.3102 and y=0.3189 in the graph of FIG. 4.
The chromaticity satisfies the safety standard in Japan for road trucking
vehicles. A color temperature of the illumination light was 6700 K. An
average color rendering index Ra was 80.3. A special color rendering
index R9 was 57.7. The Example 2 employs a higher ratio of the silicon
resin which serves as the sealing material, and a lower ratio of the
fluorescent materials, than those of the Example 1. It is considered that
the lower density of the fluorescent materials resulted in a higher
intensity of an excitation light component at 405 nm, so that the high
color temperature was obtained.

[0135]FIG. 6 is a graph showing an emission spectrum of the light
emitting part 5 of the present example. As shown in FIG. 6, this made it
possible to obtain an emission spectrum which has a sufficiently high
intensity near 510 nm which is the peak of luminosity factors in the
scotopic vision. Further, the luminous intensity at the emission spectrum
peak of the Caα-SiAlON fluorescent material which is the first
fluorescent material is higher than the luminous intensities in the
emission spectrum covering the range of not less than 540 nm but not more
than 570 nm within which range the peak of luminosity factor in the
photopic vision falls.

[0136] As compared to the Example 1, an intensity of the present example
near 510 nm is relatively higher than the luminous intensities in the
emission spectrum covering the range of not less than 540 nm but not more
than 570 nm.

[0137] As a result, employment of the white light source of the present
example as a vehicle headlamp makes it possible to realize a vehicle
headlamp which excels in obstruction visibility in night driving.

[0138] The white light source in the Example 2 is not limited to one which
is used in a completely dark place. That is, the white light source may
be used in a light environment with dim light such as early evening.

[0139] (Modification)

[0140] The above deals with, as an example of the excitation light
sources, only the laser diodes which emit laser beams at an oscillation
wavelength of 405 nm. However, excitation light sources which can be
employed in the present invention are not limited to this. For example,
the excitation light sources may be conventional light emitting diodes
which illuminate at nearly 450 nm. By employing the
Caα-SiAlON:Ce3+ fluorescent material which has an emission
peak near 510 nm, this also makes it possible to obtain a white light
source which makes it possible to realize a vehicle headlamp having an
improved obstruction visibility in the scotopic vision.

[0141] The reason why the Caα-SiAlON:Ce3+ fluorescent material
has its emission peak in a range of not less than 500 nm but not more
than 520 nm is that Ce3+ exists at a luminescence center. Therefore,
any fluorescent material can be employed as the first fluorescent
material instead of the Caα-SiAlON:Ce3+ fluorescent material,
provided that the fluorescent material has Ce3+ at its luminescence
center.

[0142] Further, the second fluorescent material may be
Sr0.8Ca0.2AlSiN3:Eu fluorescent material. The
SrCaAlSiN3:Eu (SCASN) fluorescent material has a high heat
resistance. Therefore, the light emitting part is unlikely to become
deteriorated even if the light emitting part is irradiated with a
high-power excitation light at a high light density. Further, the
SrCaAlSiN3:Eu (SCASN) fluorescent material has its emission peak
wavelength in a range of not less than 615 nm but not more than 630 nm.
Further, the emission peak wavelengths thereof are 615 nm to 630 nm.
Thus, the SCASN fluorescent material has its emission peak in a
wavelength range which is further closer to the peak of the luminosity
factor in the scotopic vision than the CASN fluorescent material having
its emission peak in the wavelength range of not less than 620 nm but not
more than 680 nm. This makes it possible to realize a vehicle headlamp
which achieves a high scotopic visibility, a high luminance, and a high
luminous flux.

[0143] Further, the first fluorescent material may be a semiconductor
nanoparticle fluorescent material containing a III-V group compound
semiconductor. In a case where the first fluorescent material is the
semiconductor nanoparticle fluorescent material, a fluorescence
wavelength varies according to a size of the nanoparticles. Therefore, in
this case, the size of the nanoparticles is adjusted so that an emission
peak falls within a range of not less than 500 nm but not more than 520
nm.

[0144] In a case where the nanoparticles have a uniform size, the
semiconductor nanoparticle fluorescent material has a sharp peak of the
emission spectrum. In a case where the nanoparticles have nonuniform
sizes in contrast, the semiconductor nanoparticle fluorescent material
has a gentle peak of the emission spectrum. Accordingly, by adjusting a
size distribution of the nanoparticles in the semiconductor nanoparticle
fluorescent material, it becomes possible to easily adjust the emission
spectrum of the light emitting part 5.

[0145] Broadly speaking, there are two methods for adjusting sizes of the
nanoparticles in the semiconductor nanoparticle fluorescent material. The
semiconductor nanoparticle fluorescent material is produced by a chemical
synthesis method. In one of the two methods for adjusting the sizes of
the nanoparticles, a process parameter (e.g., temperature and/or time) in
the chemical synthesis is changed so that a production size of the
nanoparticles is adjusted.

[0146] The other method is to classify (screen), by size, the
nanoparticles in the produced semiconductor nanoparticle fluorescent
material. The first and second methods are actually combined so as to
obtain the semiconductor nanoparticle fluorescent material having a
desired particle size.

[0147] A size of the semiconductor nanoparticles having an emission peak
in the range of not less than 500 nm but not more than 520 nm varies
depending on a material for the semiconductor nanoparticle fluorescent
material. For example, in a case where the semiconductor nanoparticle
fluorescent material is InP, the size is not less than 1.7 nm but not
more than 2.0 nm. In a case where the semiconductor nanoparticle
fluorescent material is CdSe, the size is not less than 2.0 nm but not
more than 2.2 nm.

[0148] Alternatively, the first and second fluorescent materials may be
semiconductor nanoparticle fluorescent materials. In this case, two
semiconductor nanoparticle fluorescent materials are mixed which have
respective different nanoparticle sizes.

[0149] Alternatively, the first and second fluorescent materials may be an
oxynitride fluorescent material and a semiconductor nanoparticle
fluorescent material, respectively. The oxynitride fluorescent material
and the semiconductor nanoparticle fluorescent material may be
interchanged.

[0150] The present invention does not exclude, from its technical scope,
employment of a light emitting part which contains a third fluorescent
material in addition to the first and second fluorescent materials. What
is important here is that: the first fluorescent material has its
emission peak in the range of not less than 500 nm but not more than 520
nm; accordingly, an intensity in the emission spectrum of the
illumination light is sufficiently high near 500 nm to 520 nm; and the
intensity is not lower than intensities in other wavelength ranges. As
long as the requirement is satisfied, fluorescent materials except the
first fluorescent material and the sealing material may be varied in any
way in type and ratio.

[0151] In a case where the white light source is realized as a vehicle
headlamp, the fluorescent materials are adjusted in type and ratio so
that, as described above, a white color is realized which satisfies the
safety standard for road trucking vehicles.

[0152] (Effect of Headlamp 1)

[0153] As described above, application of the technical idea of the
present invention to a vehicle headlamp makes it possible to realize the
headlamp 1 which achieves an excellent visibility at least in the
scotopic vision. Furthermore, the headlamp 1 makes it possible to obtain
white light which satisfies safety standards in Japan etc., and which has
a very high color rendering property.

[0154] The foregoing example is based on the safety standard in Japan for
road trucking vehicles. A color of the illumination light of the headlamp
1 is adjusted in accordance with a rule stipulated in a country or a
region (state or the like) in which the headlamp 1 is used.

Embodiment 2

[0155] The following describes another embodiment of the present
invention, with reference to FIG. 7. Members which are the same as those
of the Embodiment 1 are given common reference signs, and descriptions of
such members are not repeated. The present embodiment deals with a
projector-type headlamp 20.

[0156] (Arrangement of Headlamp 20)

[0157] First, the following describes an arrangement of the headlamp 20 of
the present embodiment, with reference to FIG. 7. FIG. 7 is a
cross-sectional view illustrating an arrangement of the headlamp 20 which
is a projector-type headlamp. The headlamp 20 is different from the
headlamp 1 in that the headlamp 20 is a projector-type headlamp, and
includes an optical fiber 40 instead of the light guide section 4.

[0158] As illustrated in FIG. 7, the headlamp 20 includes laser diodes 2,
aspheric lenses 3, an optical fiber (light guide section) 40, a ferrule
9, a light emitting part 5, a reflection mirror 6, a transparent plate 7,
a housing 10, an extension 11, a lens 12, a convex lens 13, and a lens
holder 8. The laser diodes 2, the optical fiber 40, the ferrule 9, and
the light emitting part 5 constitute a fundamental structure of a light
emitting device.

[0159] The headlamp 20 is a projector-type headlamp, and therefore
includes the convex lens 13. The present invention may be applied also to
another type of headlamp, such as a semi-shield beam headlamp. In this
case, the convex lens 13 may be omitted.

[0160] (Aspheric Lenses 3)

[0161] The aspheric lenses 3 are lenses for guiding laser beams
(excitation light) emitted from the laser diodes 2 so that the laser
beams enter the optical fiber 40 via light receiving ends each of which
is one of two opposite ends of the optical fiber 40. The aspheric lenses
3 are provided as many as optical fibers 40a.

[0162] (Optical Fiber 40)

[0163] The optical fiber 40 is a light guide for guiding, to the light
emitting part 5, the laser beams emitted from the laser diodes 2. The
optical fiber 40 is a bundle of a plurality of optical fibers 40a. The
optical fiber 40 has a double-layered structure, which consists of (i) a
center core and (ii) a clad which surrounds the core and has a refractive
index lower than that of the core. The core is made mainly of fused
quartz (silicon oxide), which absorbs little laser beam and thus prevents
a loss of the laser beam. The clad is made mainly of (a) fused quartz
having a refractive index lower than that of the core or (b) synthetic
resin material.

[0164] For example, the optical fiber 40 is made from quartz, and has a
core of 200 μm in diameter, a clad of 240 μm in diameter, and
numerical apertures (NA) of 0.22. Note however that a structure,
diameter, and material of the optical fiber 40 are not limited to those
described above. The optical fiber 40 can have a rectangular
cross-sectioned surface, which is perpendicular to a longitudinal
direction of the optical fiber 40.

[0165] The optical fiber 40 has a plurality of light-receiving ends for
receiving the laser beams, and has a plurality of exit end parts for
emitting the laser beams received via the plurality of light-receiving
ends. As described later, the plurality of exit end parts are positioned
by use of the ferrule 9 with respect to the laser beam-irradiated surface
(light receiving surface) of the light emitting part 5.

[0166] (Ferrule 9)

[0167] FIG. 8 is a view illustrating positional relation between the exit
end parts of the optical fibers 40a and the light emitting part 5. As
illustrated in FIG. 8, the ferrule 9 holds, in a predetermined pattern,
the plurality of exit end parts of the optical fibers 40a with respect to
the laser beam-irradiated surface of the light emitting part 5. The
ferrule 9 may have holes provided thereon in a predetermined pattern so
as to accommodate the optical fibers 40a. Alternatively, the ferrule 9
can be separated into an upper part and a lower part, on each of which
provided are bonding surface grooves for sandwiching and accommodating
the optical fibers 40a.

[0168] A material for the ferrule 9 is not particularly limited. For
example, the material is stainless steel. FIG. 8 shows three optical
fibers 40a. However, the number thereof is not limited to three. The
ferrule 9 is fixed by use of a member such as a bar-shaped member
extended from the reflection mirror 6.

[0169] The positioning of the exit end parts of the optical fibers 40a by
use of the ferrule 9 makes it possible to irradiate different parts on
the light emitting part 5 with respective parts (highest-intensity parts)
of the laser beams emitted from the plurality of optical fibers 40a which
parts are the highest in intensity in respective light intensity
distributions. The arrangement makes it possible to prevent a significant
deterioration of the light emitting part 5 which is caused by convergence
of the laser beams at one point. The exit end parts may have contact with
the laser beam-irradiated surface, or may be positioned at small
intervals.

[0170] It is not always necessary to position the exit end parts at
intervals. A bundle of the optical fibers 40a may be positioned by use of
the ferrule 9.

[0171] (Light Emitting Part 5)

[0172] The light emitting part 5 is the same as that of the Embodiment 1.
The light emitting part 5 is provided in the vicinity of a first focal
point (to be described later) of the reflection mirror 6. The light
emitting part 5 may be fixed to an end of a tubular part that extends
through a central portion of the reflection mirror 6.

[0173] (Reflection Mirror 6)

[0174] The reflection mirror 6 is, e.g., a member whose surface is coated
with a metal thin film. The reflection mirror 6 reflects light emitted
from the light emitting part 5, in such a way that the light is converged
on a focal point of the reflection mirror 6. Since the headlamp 20a is a
projector-type headlamp, a cross-sectional surface, of the reflection
mirror 6, which is in parallel with a light axis of the light reflected
by the reflection mirror 6 is basically in an elliptical shape. The
reflection mirror 6 has the first focal point and a second focal point.
The second focal point is closer to an opening of the reflection mirror 6
than the first focal point is. The convex lens 13 (to be described later)
is provided so that its focal point is in the vicinity of the second
focal point. Accordingly, the convex lens 13 projects, in a front
direction, light converged by the reflection mirror 6 at the second focal
point.

[0175] (Convex Lens 13)

[0176] The convex lens 13 converges the light emitted from the light
emitting part 5 so as to project the converged light in the front
direction from the headlamp 1. The convex lens 13 has its focal point in
the vicinity of the second focal point of the reflection mirror 6. A
light axis of the convex lens 13 extends through a substantially central
portion of the light emitting surface of the light emitting part 5. The
convex lens 13 is held by the lens holder 8, and is specified for its
relative position with respect to the reflection mirror 6. The lens
holder 8 may be formed as a part of the reflection mirror 6.

[0177] (Other Members)

[0178] The housing 10 defines a main body of the headlamp 20, and houses
the reflection mirror 6 etc. The optical fiber penetrates the housing 10.
The laser diodes 2 are provided outside the housing 10. Note here that
the laser diodes 2 generate heat when emitting laser beams. In this
regard, since the laser diodes 2 are provided outside the housing 10, the
laser diodes 2 can be efficiently cooled down. Further, in consideration
of a failure, it is preferable that the laser diodes 2 be provided in
positions where they can be easily replaced. If there is no need to take
these points into consideration, the laser diodes 2 can be housed in the
housing 10.

[0179] The extension 11 is provided in an anterior portion of a side
surface of the reflection mirror 6. The extension 11 hides an inner
structure of the headlamp 20 so that the headlamp 20 looks better, and
also strengthens connection between the reflection mirror 6 and a vehicle
body. The extension 11 is a member whose surface is coated with a metal
thin film, as is the case with the reflection mirror 6.

[0180] The lens 12 is provided on the opening of the housing 10, and seals
the headlamp 20 therein. The light emitted from the light emitting part 5
travels in a front direction from the headlamp 1 through the lens 12.

[0181] As described above, the structure of the headlamp 1 may be varied
in any wise. What is important for the present invention is that the
light emitted from the light emitting part 5 sufficiently contains light
which achieves a high visibility at least in the scotopic vision.

[0182] As described above, the illuminating device of the present
invention is preferably arranged such that the first fluorescent material
contains Ce3+ as its luminescence center.

[0183] According to the arrangement, the first fluorescent material
containing Ce3+ as its luminescence center is employed as the first
fluorescent material. This makes it possible to easily generate light
which has its emission spectrum peak in the range of not less than 500 nm
but not more than 520 nm, and which has a very broad emission spectrum
covering a wavelength near the peak of the luminosity factor in the
photopic vision.

[0184] This makes it possible to realize an illuminating device whose
brightness cannot be felt by a user to be discontinuous in a case where a
luminosity factor varies from early evening (photopic vision) in which
dim light still remains to dark night (scotopic vision). Examples of the
fluorescent material containing Ce3+ as its luminescence center
encompass Caα-SiAlON:Ce3+ fluorescent material.

[0185] Further, the illuminating device of the present invention is
preferably arranged such that the second fluorescent material has its
emission spectrum peak in a range of not less than 600 nm but not more
than 680 nm.

[0186] According to the arrangement, the fluorescence of the second
fluorescent material has its emission spectrum peak in the range of not
less than 600 nm but not more than 680 nm. Since the fluorescence of the
first fluorescent material has its emission spectrum peak in the range of
not less than 500 nm but not more than 520 nm, it is possible to easily
adjust, within a range of white colors, a color of the light to be
emitted from the light emitting part, by changing the ratio between the
first and second fluorescent materials.

[0187] Further, the illuminating device of the present invention is
preferably arranged such that the excitation light source emits
excitation light having a wavelength of not less than 400 nm but not more
than 420 nm.

[0188] By combining the first fluorescent material (emission peak
wavelength is not less than 500 nm but not more than 520 nm) with the
excitation light source for emitting excitation light having a wavelength
in the range of not less than 400 nm but not more than 420 nm, it becomes
possible to expand the range of options to choose a second fluorescent
material which is required to realize an illuminating device having a
light emitting part for emitting white light. Specifically, it becomes
possible to employ, as the second fluorescent material, a fluorescent
material having its emission spectrum peak in the range of not less than
600 nm but not more than 680 nm.

[0189] Further, the illuminating device of the present invention is
preferably arranged such that the first fluorescent material is
Caα-SiAlON (silicon aluminum oxynitride):Ce fluorescent material.

[0190] The Caα-SiAlON (silicon aluminum oxynitride):Ce fluorescent
material has a high heat resistance. Therefore, according to the
arrangement, the light emitting part is unlikely to become deteriorated
even if the light emitting part is irradiated with a high-power
excitation light at a high light density. This makes it possible to
realize an illuminating device which achieves a high luminance and a high
luminous flux.

[0191] Further, the illuminating device of the present invention is
preferably arranged such that the first fluorescent material is a
nanoparticle fluorescent material containing a III-V group compound
semiconductor.

[0192] In a case where the nanoparticles have a uniform size, the
semiconductor nanoparticle fluorescent material has a sharp peak of the
emission spectrum. In a case where the nanoparticles have nonuniform
sizes in contrast, the semiconductor nanoparticle fluorescent material
has a gentle peak of the emission spectrum. Therefore, according to the
arrangement, it becomes possible to easily adjust the emission spectrum
of the light emitting part, by adjusting a size distribution of the
nanoparticles in the first fluorescent material.

[0193] Further, the illuminating device of the present invention is
preferably arranged such that the second fluorescent material is
CaAlSiN3:Eu fluorescent material.

[0194] The CaAlSiN3:Eu (CASN) fluorescent material has a high heat
resistance. Therefore, according to the arrangement, the light emitting
part is unlikely to become deteriorated even if the light emitting part
is irradiated with a high-power excitation light at a high light density.
This makes it possible to realize an illuminating device which achieves a
high luminance and a high luminous flux.

[0195] Further, the illuminating device of the present invention is
preferably arranged such that the second fluorescent material is
Sr0.8Ca0.2AlSiN3:Eu fluorescent material.

[0196] The SrCaAlSiN3:Eu (SCASN) fluorescent material has a high heat
resistance. Therefore, according to the arrangement, the light emitting
part is unlikely to become deteriorated even if the light emitting part
is irradiated with a high-power excitation light at a high light density.
Furthermore, the SrCaAlSiN3:Eu (SCASN) fluorescent material has its
emission peak wavelength in a range of not less than 615 but not more
than 630 nm. The emission peak wavelength is further close to the peak of
the luminosity factor in the scotopic vision. This makes it possible to
realize an illuminating device which achieves a high visibility in the
scotopic vision, a high luminance, and a high luminous flux.

[0197] Further, a vehicle headlamp of the present invention includes the
illuminating device, a color of light which is emitted from the light
emitting part being a white color which falls within a legally-stipulated
range of colors of light of vehicle headlamps.

[0198] In countries such as Japan and the US, it is required by law to
employ, as a color of light of a vehicle headlamp, a white color having a
chromaticity in a predetermined range.

[0199] According to the arrangement, the second fluorescent material has
an emission spectrum peak which is different from that of the first
fluorescent material; and a fluorescence color of the second fluorescent
material and the ratio between the first and second fluorescent materials
in the light emitting part are adjusted so that a fluorescence color of
light which is emitted from the light emitting part when the light
emitting part is irradiated with the excitation light is a white color
which falls within the range of colors of light of vehicle headlamps
which range is legally stipulated in a country or a region (state or the
like) in which the vehicle headlamp is used.

[0200] This makes it possible to generate light which has its emission
spectrum peak in the range of not less than 500 nm but not more than 520
nm, and which has a chromaticity in the legally-stipulated range. In
addition, it is possible to realize a vehicle headlamp having an improved
visibility at least in the scotopic vision.

[0201] The invention being thus described, it will be obvious that the
same way may be varied in many ways. Such variations are not to be
regarded as a departure from the spirit and scope of the invention, and
all such modifications as would be obvious to one skilled in the art are
intended to be included within the scope of the following claims.

INDUSTRIAL APPLICABILITY

[0202] The present invention is applicable to an illuminating device and a
headlamp which are used in a case where it is necessary to improve
visibility of an object (particularly in a dark place), particularly to a
vehicle headlamp.